Meanwhile Syrian armed forces have announced the formation of an anti-terrorism force called Fifth Corps.

The Syrian army did not specify where the force would be deployed.

Moscow's military backing of the Syrian government, with intensive aerial bombings and long-range missiles fired from the Mediterranean, has dramatically turned the fortunes of the Syrian government - emboldening ground advances on multiple fronts, including in the besieged rebel-held parts of Aleppo city.

State TV on Tuesday aired announcements urging armed opposition groups to allow civilians to exit the besieged enclave of Aleppo through government-designated corridors.

Another warning urged residents to co-operate with government forces. A third called on residents to avoid going out in the streets except in "dire need" and to stay clear of areas where armed groups operate.

On Sunday, the Syrian government dismissed a UN peace proposal to establish limited autonomy in rebel-held eastern Aleppo.

The offensive was accompanied by pro-government troops pushing their way into neighbourhoods on the edges of eastern Aleppo.

Fighting on the southern edge, in the Sheik Saeed neighbourhood, intensified on Tuesday. A major rebel group, Ahrar al-Sham, said one of its leading commanders was killed as they repelled advances by government troops.

On Tuesday, Assad received a Russian delegation in Damascus, headed by the Russian deputy prime minister, in a show of close ties between the two governments in the face of international criticism.